The present invention relates generally to structural improvement of induction type electric machines and, more particularly, to a method of assembling an induction rotor.
An induction motor is an asynchronous electric machine powered by alternating current (AC), where such power is induced in a rotor via electromagnetic induction. For example, polyphase AC currents may be provided to stator windings structured to create a rotating magnetic field that induces current in conductors of a rotor, whereby interaction between such induced currents and the magnetic fields causes the rotor to rotate. Induction motors may have any number of phases. An induction motor may operate as a generator or traction motor, for example, when driven at a negative slip.
Rotors of induction motors may conventionally include a cage such as a squirrel cage having parallel axial or skewed conductor bars of copper or aluminum extending between opposite rotor ends and positioned at radially outward locations along the circumference of the rotor. The rotor may have a substantially cylindrical iron core formed as a stack of individual laminated disks, for example disks of a silicon steel material. Each core disk may have axial slots for passing the copper or aluminum bars there-through when the slots are in alignment with one another in a lamination stack. Distal ends of individual conductor bars may be structurally supported and in electrical communication with one another by connection of the respective bar ends to one or more end rings disposed at each rotor end.
Due to the high costs associated with permanent magnet electric motors, electric machines for many different applications are being redesigned to utilize induction rotors. However, conventional induction rotors may have a reduced number of applications due to poor mechanical properties of the chosen material and/or due to inconsistent assembly methods, especially when structural weakness is exacerbated by the size and speed of the rotor. When an induction motor is utilized in a given application such as automotive, the rotor must tolerate high speed rotation and associated large centrifugal force. In addition, high temperatures, potential metal fatigue, and other factors may aggregate to cause structural breakdown resulting in damage or deformation of the rotor. For example, an induction rotor generates higher temperatures within the rotor itself, further reducing mechanical and structural integrity.
There are various conventional techniques that may be used for assembling induction rotors. For example, conventional induction machines may utilize varying grades of aluminum or copper in die-casting the end rings/plates and the conductor bars of the cage as an integral unit. However, conventional die-cast induction rotors may have a reduced number of applications due to poor mechanical properties of the chosen die-cast material and due to problems related to manufacturing. Depending on the grade, the cast material strength may vary significantly. Another conventional induction rotor assembly technique may include forming individual conductor bars, forming two end rings having slots/channels corresponding to the axial slots of the lamination stack, inserting the bars through the axial slots, positioning the respective end rings at the opposite axial ends of the rotor so that the conductor bars pass through the end rings, pressing the end rings axially toward one another, and then welding the end portions of the conductor bars to the end rings. Such welding of conductor bars may produce inconsistent results and poor contact between the end rings and the conductor bars. A further conventional technique for assembling induction rotors may substitute a heading operation for the welding of conductor bars. In such a heading process, the protruding ends of the conductor bars are compressed and flattened against the respective exterior axial surfaces of the end rings. Structural problems may result from a heading operation. After being impacted in an axially inward direction, the compressed conductor bars become self-biasing in an axially outward direction and, over time, such conductor bars expand and become loose with respect to the end ring slots.